Fixes wrong cursor position after completion overwrite

When completion overwrites is enabled the cursor moved to a different
position when a completion that needs an import is inserted. It was
just forgotten to add the number of signs the import contains to the
new offset of the cursor.

Fixes #1001797
Merge pull request #493 from sschaef/t-1001797-move-cursor-on-completion-overwrite

Fixes wrong cursor position after completion overwrite